1. Field
The present invention relates generally to data communication, and more specifically to techniques for controlling transmit power in multi-channel communication systems (e.g., multiple-input, multiple-output (MIMO) systems) that utilize partial channel-state information (CSI).
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input, multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. For example, an independent data stream may be transmitted on each of the NS spatial subchannels to increase system throughput.
Multiple data streams may be transmitted on the spatial subchannels using channel-state information (CSI), which is descriptive of the characteristics of the MIMO channel. CSI may be categorized as either “full CSI” or “partial CSI”. Full CSI includes sufficient characterization (e.g., amplitude and phase) of the propagation path between each transmit-receive antenna pair in a (NR×NT) MIMO matrix. Full CSI may not be available or practical for many MIMO systems. Partial CSI may comprise, for example, the signal-to-noise-and-interference ratios (SNRs) of the spatial subchannels, which may be estimated by detecting the data streams and/or pilots transmitted on these subchannels. Each data stream may then be coded and modulated in accordance with a particular coding and modulation scheme selected based on the subchannel's SNR.
The spatial subchannels of a MIMO system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different SNRs for a given amount of transmit power. Consequently, the data rates that may be supported by the spatial subchannels may be different from subchannel to subchannel. Moreover, the channel conditions typically vary with time. As a result, the data rates supported by the spatial subchannels also vary with time.
A key challenge in a MIMO system is the determination of the transmit powers to use for the data transmissions on the spatial subchannels based on the channel conditions. The goal of this transmit power control should be to maximize spectral efficiency while meeting other system objectives, such as achieving a particular target frame error rate (FER) for each data stream, minimizing interference, and so on.
In a practical communication system, there may be an upper limit on the data rate that may be used for any given data stream. For example, a set of discrete data rates may be supported by the system, and the maximum data rate from among these discrete data rates may be considered as the maximum spectral efficiency for any given data stream. In such a system, utilizing more transmit power than necessary for each data stream to achieve the target FER at the maximum data rate would result in an ineffective use of the additional transmit power. Even though the excess transmit power may result in a lower FER, this improvement in FER may not be considered substantial since the target FER has already been achieved. The excess transmit power for a given data stream may result in additional interference to other data streams, which may then degrade the performance of these data streams.
There is therefore a need in the art for techniques to control the transmit power of the data streams in a MIMO system utilizing partial CSI.